1. Field of the Invention
The present invention relates to a process for the separation of ethylbenzene and styrene by distillation. More particularly, it is drawn to the recovery of ethylbenzene and styrene from the crude product resulting from ethylbenzene dehydrogenation, utilizing the advantages of heat pump/vapor compression in a distillation process.
2. Discussion of the Background
Styrene is by far the most commercially important unsaturated aromatic monomer (see Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 21, Third Edition (1983) pp. 770-801, incorporated by reference). The dominant manufacturing technique is the dehydrogenation of ethylbenzene, which provides a crude product composed mainly of styrene and ethylbenzene. Separation of this crude product by distillation is difficult because of the similar boiling points of styrene and ethylbenzene. Distillation is further complicated by the presence of impurities such as toluene, benzene, water, and nitrogen-containing species, and by practices such as the addition of vinyl aromatic polymerization inhibitors.
A great many distillation systems are known (see ibid, Vol. 7, pp. 849-891; ibid, Supplement, pp. 806-845; King, C. J., Separation Processes, Second Edition, McGraw-Hill, N.Y., 1980; incorporated by reference). Since the separation of ethylbenzene and styrene is both uniquely difficult and is of great economic importance, many distillation processes have been proposed specifically therefore. Such a process is disclosed, for example, in Bulletin VT 1673/74, May 1982, page 3, 12.4.1, Separation of Ethylbenzene/Styrene, Sulzer Company. However, control of the distillation column (crude styrene column) is not disclosed.
EP-B 0 160 553 teaches a further development of the process of the Sulzer Company. In this case, the vapor of the low boilers, including ethylbenzene, is withdrawn from the head of a distillation column and divided into two parts or streams. A first stream is passed in a known manner through a "preheater" in which the stream is superheated and then to a compressor for adiabatic compression. A second stream is passed to a condenser. The heat of condensation of the second stream is apparently lost to the system in the condenser, and the distillation column is controlled in a complicated manner. Although the distillation is highly stable (e.g. mass transfer is maintained at a constant rate and purity), it is costly.
Thus, there is a need for an efficient process of separating a mixture comprising ethylbenzene and styrene that is stable, easily controlled, and yet less costly.